1. Field of the Invention
The present invention relates generally to the field of parametric sound systems. More particularly, the present invention relates to a method of producing a parametric ultrasonic output wave to be decoupled in air to create an decoupled audio wave that more closely corresponds to the audio input signal.
2. Related Art
Audio reproduction has long been considered a well-developed technology. Over the decades, sound reproduction devices have moved from a mechanical needle on a tube or vinyl disk, to analog and digital reproduction over laser and many other forms of electronic media. Advanced computers and software now allow complex programming of signal processing and manipulation of synthesized sounds to create new dimensions of listening experience, including applications within movie and home theater systems. Computer generated audio is reaching new heights, creating sounds that are no longer limited to reality, but extend into the creative realms of imagination.
Nevertheless, the actual reproduction of sound at the interface of electro-mechanical speakers with the air has remained substantially the same in principle for almost one hundred years. Such speaker technology is clearly dominated by dynamic speakers, which constitute more than 90 percent of commercial speakers in use today. Indeed, the general class of audio reproduction devices referred to as dynamic speakers began with the simple combination of a magnet, voice coil and cone, driven by an electronic signal. The magnet and voice coil convert the variable voltage of the signal to mechanical displacement, representing a first stage within the dynamic speaker as a conventional multistage transducer. The attached cone provides a second stage of impedance matching between the electrical transducer and air envelope surrounding the transducer, enabling transmission of small vibrations of the voice coil to emerge as expansive compression waves that can fill an auditorium. Such multistage systems comprise the current fundamental approach to reproduction of sound, particularly at high energy levels.
A lesser category of speakers, referred to generally as film or diaphragmatic transducers, rely on movement of a large emitter surface area of film (relative to audio wavelength) that is typically generated by electrostatic or planar magnetic driver members. Although electrostatic speakers have been an integral part of the audio community for many decades, their popularity has been quite limited. Typically, such film emitters are known to be low-power output devices having applications appropriate only to small rooms or confined spaces. With a few exceptions, commercial film transducers have found primary acceptance as tweeters and other high frequency devices in which the width of the film emitter is equal to or less than the propagated wavelength of sound. Attempts to apply larger film devices have resulted in poor matching of resonant frequencies of the emitter with sound output, as well as a myriad of mechanical control problems such as maintenance of uniform spacing from the stator or driver, uniform application of electromotive fields, phase matching, frequency equalization, etc
As with many well-developed technologies, advances in the state of the art of sound reproduction have generally been limited to minor enhancements and improvements within the basic fields of dynamic and electrostatic systems. Indeed, substantially all of these improvements operate within the same fundamental principles that have formed the basics of well-known audio reproduction. These include the concept that (i) sound is generated at a speaker face, (ii) based on reciprocating movement of a transducer (iii) at frequencies that directly stimulate the air into the desired audio vibrations. From this basic concept stems the myriad of speaker solutions addressing innumerable problems relating to the challenge of optimizing the transfer of energy from a dense speaker mass to the almost massless air medium that must propagate the sound.
A second fundamental principle common to prior art dynamic and electrostatic transducers is the fact that sound reproduction is based on a linear mode of operation. In other words, the physics of conventional sound generation relies on mathematics that conform to linear relationships between absorbed energy and the resulting wave propagation in the air medium. Such characteristics enable predictable processing of audio signals, with an expectation that a given energy input applied to a circuit or signal will yield a corresponding, proportional output when propagated as a sound wave from the transducer.
In such conventional systems, maintaining the air medium in a linear mode is extremely important. If the air is driven excessively into a nonlinear state, severe distortion occurs and the audio system is essentially unacceptable. This nonlinearity occurs when the air molecules adjacent the dynamic speaker cone or emitter diaphragm surface are driven to excessive energy levels that exceed the ability of the air molecules to respond in a corresponding manner to speaker movement. In simple terms, when the air molecules are unable to match the movement of the speaker so that the speaker is loading the air with more energy than the air can dissipate in a linear mode, then the a nonlinear response occurs, leading to severe distortion and speaker inoperability. Conventional sound systems are therefore built to avoid this limitation, ensuring that the speaker transducer operates strictly within a linear range.
Parametric sound systems, however, represent an anomaly in audio sound generation. Instead of operating within the conventional linear mode, parametric sound can only be generated when the air medium is driven into a nonlinear state. Within this unique realm of operation, audio sound is not propagated from the speaker or transducer element. Instead, the transducer is used to propagate carrier waves of high-energy ultrasonic bandwidth beyond human hearing. The ultrasonic wave therefore functions as the carrier wave, which can be modulated with audio input that develops sideband characteristics capable of decoupling in air when driven to the nonlinear condition. In this manner, it is the air molecules and not the speaker transducer that will generate the audio component of a parametric system. Specifically, it is the sideband component of the ultrasonic carrier wave that energizes the air molecule with audio signal, enabling eventual wave propagation at audio frequencies.
Another fundamental distinction of a parametric speaker system from that of conventional audio is that high-energy transducers as characterized in prior art audio systems do not appear to provide the necessary energy required for effective parametric speaker operation. For example, the dominant dynamic speaker category of conventional audio systems is well known for its high-energy output. Clearly, the capability of a cone/magnet transducer to transfer high energy levels to surrounding air is evident from the fact that virtually all high-power audio speaker systems currently in use rely on large dynamic speaker devices. In contrast, low output devices such as electrostatic and other diaphragm transducers are virtually unacceptable for high power requirements. As an obvious example, consider the outdoor audio systems that service large concerts at stadiums and other outdoor venues. It is well known that massive dynamic speakers are necessary to develop direct audio to such audiences. To suggest that a low power film diaphragm might be applied in this setting would be considered foolish and impractical.
Yet in parametric sound production, the present inventors have discovered that a film emitter will outperform a dynamic speaker in developing high power, parametric audio output. Indeed, it has been the general experience of the present inventors that efforts to apply conventional audio practices to parametric devices will typically yield unsatisfactory results. This has been demonstrated in attempts to obtain high sound pressure levels, as well as minimal distortion, using conventional audio techniques. It may well be that this prior art tendency of applying conventional audio design to construction of parametric sound systems has frustrated and delayed the successful realization of a commercial parametric sound. This is evidenced by the fact that prior art patents on parametric sound systems have utilized high energy, multistage bimorph transducers comparable to conventional dynamic speakers. Despite widespread, international studies in this area, none of these parametric speakers were able to perform in an acceptable manner.
In summary, whereas conventional audio systems rely on well accepted acoustic principles of (i) generating audio waves at the face of the speaker transducer, (ii) based on a high energy output device such as a dynamic speaker, (iii) while operating in a linear mode, the present inventors have discovered that just the opposite design criteria are preferred for parametric applications. Specifically, effective parametric sound is effectively generated using (i) a comparatively low-energy film diaphragm, (ii) in a nonlinear mode, (iii) to propagate an ultrasonic carrier wave with a modulated sideband component that is decoupled in air (iv) at extended distances from the face of the transducer. In view of these distinctions, it is not surprising that much of the conventional wisdom developed over decades of research in conventional audio technology is simply inapplicable to problems associated with the generation parametric sound.
One specific area of conventional audio technology that is largely inapplicable to transducer design is in the field of pre-processing an electrical signal prior to its emission from a transducer. While many traditional signal processing techniques are well known as means to enhance the acoustical output of a conventional audio speaker, these techniques are largely inadequate when applied to the field of parametric sound systems. This is because it has been unnecessary for traditional signal processing techniques to account for the non-linear distortion that is often created when parametric ultrasonic waves decouple in air as a non-linear medium to form a decoupled audio wave. Conventional audio technology would simply not need to worry about the non-linearity of air, since they are purposely built such that the air will remain in a substantially linear range. While some of the traditional signal processing techniques may be applied to parametric audio systems, and may even enhance the decoupled audio wave to some degree, these traditional techniques are largely inadequate when it comes to eliminating non-linear distortion caused by the non-linearity of air in which parametric speakers operate.
What is needed is a system and method for substantially accounting for and eliminating the non-linear distortion that is often created when parametric ultrasonic waves decouple in air as a non-linear medium to form a decoupled audio wave.